Multipurpose hot melt adhesive

ABSTRACT

A multipurpose hot melt adhesive comprising endblock resin in combination with a radial or linear SBS copolymer having a solution viscosity greater than 1000 cPs at 25% in toluene suitable for use as both a construction and elastic attachment adhesive in disposable products is disclosed.

This application is a continuation-in-part of prior application Ser. No.09/219,938 filed on Dec. 23, 1998, now U.S. Pat. No. 6,232,391.

FIELD OF THE INVENTION

The present invention is directed to a multipurpose hot melt adhesive,which is suitable for use in disposable articles as both a constructionand elastic attachment adhesive, comprising endblock resin incombination with a radial or linear styrene-butadiene-styrene or “SBS”block copolymer having a solution viscosity greater than 1000 cPs at 25%in toluene.

BACKGROUND OF THE INVENTION

In the production of disposable articles, hot melt adhesives aretypically extruded at elevated temperatures (about 250° F. to 350° F.)directly onto a work piece, typically a polyethylene or polypropylenefilm, a nonwoven fabric, an absorbent material, a tissue, or a filmwhich can then be bonded to another nonwoven fabric, absorbent material,tissue, or film using the hot melt adhesive. Application of the adhesivemay be extruded by fine line, multi-dot, multi-line methods or spraytechniques. The hot melt adhesive is used to laminate layers or attachelastic, particularly for waist or leg band closures for disposablediapers; elastic attachment also results in laminating.

Since in the assembly of disposable articles a variety of materials arebonded under a wide range of conditions, to optimize performanceseparate adhesives have evolved for use in the manufacture of disposablearticles. This is particularly true in elastic attachment and inlaminate construction. Construction requires an adhesive withcontrollable, relatively low viscosity, long open time, and sufficientbonding strength to maintain the mechanical integrity of the laminate.However, to bond elastic materials to substrates, a different adhesivewhich exhibits high creep resistance to ensure that the elastic, whenunder stress, does not move relative to the surfaces of the substratesor become partially or fully detached, is used.

The use of separate adhesives for laminate construction and for elasticattachment purposes, each having different formulas and properties,increases disposable article manufacturing complexity and can reduceproductivity. Additionally, if multiple adhesives are required inventoryand storage problems are increased. If the incorrect adhesive is usedfor elastic bonding it can lead to bond failure. Also, the applicatorcan become plugged. Such problems can lead to inferior products, lostproduction or both.

Accordingly a substantial need exists for a single adhesive havingproperties rendering the adhesive applicable to both lamination andelastic bonding applications. Such adhesives are known as multipurposeadhesives.

Construction adhesives are soft (tacky), have high peel strength, longopen time and low cohesive strength. Elastic attachment adhesives arestiffer (not as tacky), high in cohesive strength, and shorter in opentime, than construction adhesives. For multipurpose applications, inparticular, elastic attachment, the balance of stiffness, cohesivestrength, and open time is required for good performance.

Styrene-isoprene-styrene or “SIS” block copolymers are commonly used forelastic attachment in disposable products. SIS is chosen because, whencompared to other block copolymers, for the same melt index and rubbercontent, SIS polymers provide a higher molecular weight and softeradhesive products. Adhesives used for elastic attachment need to containhigh amounts of rubber and endblock resin to obtain adequate cohesion.For example, U.S. Pat. No. 5,149,741 to Alper discloses elasticattachment adhesives comprising 35 parts of SIS copolymer, incombination with 10 parts endblock resin.

As used herein, the “midblock” of the polymer refers to polymeric blockswhich are substantially aliphatic. As will be discussed below, “midblockresin” refers to a tackifier which is compatible with the midblock ofthe polymer. “Endblock” of the polymer refers to polymeric blocks whichare substantially aromatic. “Endblock resins”, as will be discussed indetail below, are substantially aromatic and compatible with theendblock of the polymer.

At similar polymer content, melt index, and endblock resin level, acorresponding SBS-based adhesive is not as pressure sensitive as SIS dueto high stiffness. The higher stiffness results from the lowerentanglement molecular weight (higher entanglement density) of butadieneversus isoprene. U.S. Pat. No. 5,071,571 (Malcolm) discloses an adhesivefor elastic attachment comprising low content of a very high molecularweight SBS copolymers.

Endblock resins are commonly used with SIS to improve cohesive strengthin pressure sensitive adhesives. Endblock resins are not commonly usedwith SBS because it is difficult to ensure that the resin actuallyreaches the endblock due to the higher polarity of the butadienemidblock compared to isoprene midblock, which therefore tends tosolubilize the endblock resin in the midblock to a large extent. Inaddition, endblock resins are not commonly used in SBS-based elasticattachment adhesives. U.S. Pat. No. 4,944,993 (Raykovitz) discloses lowmolecular weight, radial SBS polymers comprising a styrene contentgreater than 35% with endblock, and their use in construction andelastic attachment adhesives.

Endblock resins are effective only when they associate with the styreneportion of the block copolymer chain. Due to the closeness in solubilityparameter of butadiene and styrene, the resin associates with themidblock or butadiene portion in low molecular weight SBS copolymers.However, in high molecular weight SBS copolymers, the longer butadienechain does not favor this association to the same extent. This effectcan be demonstrated by rheology curves, where the Tg of the butadieneblock can be followed. For example, when 10 parts midblock resin arereplaced by endblock resin, the midblock Tg should, in theory, decrease.The higher the midblock Tg after such a substitution, the more theendblock resin is in the midblock, instead of the endblock, of therubber. By using a high molecular weight rubber at low levels, incombination with endblock resin, in accordance with the presentinvention, an adhesive with high cohesive strength can be attained dueto the association of the endblock resin with the styrene block of therubber. In addition, because lower amount of rubber is used, thestiffness is low and hence, the tack is high; for the same reason, opentime is still high enough for multipurpose applications.

Therefore, unexpectedly, it has been found, in accordance with thepresent invention, that a multipurpose adhesive, for use in both elasticattachment and construction, can be prepared from a high molecularweight radial or linear SBS copolymer, without a high styrene content,and endblock resins. The adhesives of the present invention have bothgood cohesive strength (elastic attachment) and long open time and lowstiffness, for good lamination bonds. The high molecular weight SBScopolymer allows use of a lower amount of the copolymer providing asofter, more pressure sensitive adhesive, with longer open time, whencompared to use of a low molecular weight SBS copolymer. In addition,the presence of the endblock resin provides for a higher cohesion, atlower viscosity, when compared to adding more copolymer to the system toobtain the same increased level of cohesion.

The present invention is directed to hot melt adhesive compositionssuitable for disposable constructions comprising an endblock resin and ahigh molecular weight styrene-butadiene block copolymer having amolecular weight such that at 25% in toluene, the viscosity is greaterthan 1000 cPs.

SUMMARY OF THE INVENTION

It has been found, in accordance with the present invention, that theuse of endblock resin with high molecular weight rubber polymersprovides a multipurpose hot melt adhesive which is suitable for use asboth a construction and elastic attachment adhesive. Specifically, ithas been found that a multiple-purpose adhesive having a high level ofcreep resistance, bond strength, and a low viscosity profile can beprepared comprising endblock resin in combination with a radial orlinear SBS copolymer having a solution viscosity greater than 1000 cPsat 25% in toluene. The adhesive of the present invention will have along open time, low stiffness, and good cohesive strength, allproperties qualifying the adhesive as an effective multipurposeadhesive.

DESCRIPTION OF THE INVENTION

The present invention is directed to a multipurpose hot melt adhesivecomprising: an SBS polymer with molecular weight such that at 25% intoluene, the viscosity is greater than 1000 cPs and an endblock resin.

The polymers useful in the hot melt adhesive of the present inventionare block or multi-block copolymers having one of the following generalconfigurations:

(A-B)_(n)-A or (AB)_(n)-X or (A-B)_(n)

wherein X is a multivalent coupling agent with functionality of two ormore, and polymer blocks A are non-elastomeric polymer blocks andpolymer blocks B are elastomeric polymer blocks of butadiene which hasnot been hydrogenated. Variable “n” is an integer equal to, or greaterthan, one. Copolymers useful in the present invention may be linear orradial. With radial copolymers, the functionality of X is three or more.Preferably the copolymer is linear. Some level of diblock copolymer, AB,may be present by design or due to incomplete coupling of the AB arms.Diblock is beneficial for increasing tack, peel and open time, but thismust be counterbalanced with its effect of lowering cohesive strength.Diblock level will in general be below 50%, preferably less than 30%.

Examples of multivalent coupling agents, “X”, include dibromoethane withfunctionality of 2; trisnonylphenyl phosphite and trichloromethylsilane,both with functionality of 3; and tetrachlorosilane with functionalityof 4.

The non-elastomeric blocks A may comprise homopolymers or copolymers ofvinyl monomers such as vinyl arenes, vinyl pyridines, vinyl halides andvinyl carboxylates, as well as acrylic monomers such as acrylonitrile,methacrylonitrile, esters of acrylic acids, etc. Monovinyl aromatichydrocarbons include styrene, vinyl toluene, vinyl xylene, ethyl vinylbenzene as well as dicyclic monovinyl compounds such as vinylnaphthalene and the like. Other non-elastomeric polymer blocks may bederived from alpha olefins, alkylene oxides, acetals, urethanes, etc.Styrene is preferred, in an amount comprising less than 35 weightpercent of the total copolymer composition, more preferably 25 to 32weight percent, however it is most preferred that the least amount ofstyrene possible is used.

The elastomeric block component, B, making up the remainder of thecopolymer is butadiene which has not been hydrogenated.

Most preferred for use herein are the linear A-B-A triblock copolymerswhere the elastomeric block is butadiene and the non-elastomeric blockis styrene, and wherein the copolymer has a molecular weight such thatits solution viscosity at 25% in toluene is greater than 1000 cPs,preferably greater than 2000, most preferably about 4000 cPs.

Typical of the rubbery block copolymers useful herein are thepolystyrene-polybutadiene-polystyrene. Depending on the polymerizationconditions, the polybutadiene midblock will contain different ratios ofcis-1,4; trans-1,4; and 1,2 addition. Higher levels of 1,2 addition maybe desirable to lower the viscosity for a given molecular weight. Thesecopolymers may be prepared using methods familiar to one of ordinaryskill in the art. Alternatively, these polymers may be obtained fromShell Chemical Co. under the tradenames Kraton D1101, with a styrenecontent of 31% and viscosity at 25% in toluene of 4000 cPs and KratonD1184 with a styrene content of 30% and a viscosity at 25% in toluene of20,000 cPs. Examples of other commercially available copolymers includeSOLT 6302, 30% styrene, viscosity at 25% in toluene of 4000 cPsavailable from EniChem Americas (Agip USA Inc.); and DPX 563 withstyrene content of 31% and viscosity in toluene at 25% of 7970 cPsavailable from Dexco.

Blends of these styrene containing copolymers with up to about 25%,preferably less than 10%, of other compatible non-functionalized blockcopolymers may also be employed. By the term “non-functionalized” ismeant block copolymers which are not chemically modified so as tocontain functional groups such as epoxy, anhydride, silane, sulfonate,amide or the like on the copolymer backbone. Useful compatiblecopolymers include those prepared using isoprene or butadieneelastomeric midblocks, whether hydrogenated or not, such as thoseavailable from Shell under the Kraton trademark. Particularly useful isthat designated Kraton 1119.

While the optimum amounts of the copolymer used in the adhesive willvary depending on the end use application, the copolymer will generallybe present in the adhesive formulation at a level less than 35%,generally greater than 5%, more often greater than 10%, and preferablyabout 15 to 20% by weight, most preferably about 18% by weight. Sincethe copolymer used in the hot melt adhesive of the present invention isof high molecular weight, only a small amount needs to be used,resulting in a long open time and soft product.

The hot melt adhesive compositions of the present invention alsocomprises a solid tackifier which is compatible with the midblock of theSBS copolymer. Representative resins include the C₅/C₉ hydrocarbonresins, synthetic polyterpenes, rosin, rosin esters, natural terpenes,and the like. More particularly, the useful tackifying resins includeany compatible resins or mixtures thereof such as (1) natural andmodified rosins including gum rosin, wood rosin, tall oil rosin,distilled rosin, hydrogenated rosin, dimerized rosin, and polymerizedrosin; (2) glycerol and pentaerythritol esters of natural and modifiedrosins, including the glycerol ester of pale, wood rosin, the glycerolester of hydrogenated rosin, the glycerol ester of polymerized rosin,the pentaerythritol ester of hydrogenated rosin, and thephenolic-modified pentaerythritol ester of rosin; (3) copolymers andterpolymers of natural terpenes, such as styrene/terpene and alphamethyl styrene/terpene; (4) polyterpene resins generally resulting fromthe polymerization of terepene hydrocarbons, such as the bicyclicmonoterpene known as pinene, in the presence of Friedel-Crafts catalystsat moderately low temperatures; also included are the hydrogenatedpolyterpene resins; (5) phenolic modified terpene resins andhydrogenated derivatives thereof such, for example, as the resin productresulting from the condensation, in an acidic medium, of a bicyclicterpene and a phenol; (6) aliphatic petroleum hydrocarbon resinsresulting from the polymerization of monomers consisting primarily ofolefins and diolefins; also included are the hydrogenated aliphaticpetroleum hydrocarbon resins; and (7) cyclic petroleum hydrocarbonresins and the hydrogenated derivatives thereof. Mixtures of two or moreof the above described tackifying resins may be required for someformulations. Also included are the cyclic or acylic C₅ resins andaromatic modified acyclic or cyclic resins. Preferred is aromaticmodified cyclic or acyclic C₅ resin.

A preferred tackifier is a C₅/C₉ hydrocarbon resin derived frompetroleum with a Ring and Ball softening point between 100 to 105° C.The tackifiers, also referred to as “midblock resins”, are generallypresent in the adhesive compositions in an amount greater than theamount of the block copolymer. Within this range, amounts of 30 to 70%by weight of the composition, preferably 50 to 65 weight percent areutilized.

The present invention also includes 2 to 30 weight percent of anendblock resin which is substantially aromatic. Examples of suchendblock resins can be prepared from any substantially aromatic monomershaving a polymerizable unsaturated group. Typical examples of sucharomatic monomers include the styrenic monomers, styrene, alphamethylstyrene, vinyl toluene, methoxy styrene, tertiary butyl styrene,chlorostyrene, etc., coumarone, indene monomers including indene, andmethyl indene. The aromatic endblock resin is preferably present inamounts of 5 to 20 weight percent. Preferred is HERCOLITE 240 orKRISTALEX 5140, both of which are alpha methyl styrene resins availablefrom Hercules, Inc.

The hot melt adhesive of the present invention also comprises 0 to 30,preferably 5 to 20, weight percent of an oil diluent. Suitableplasticizing or extending oils or liquid tackifiers include olefinoligomers and low molecular weight polymers as well as vegetable andanimal oil and their derivatives. The petroleum derived oils which maybe employed are relatively high boiling materials containing only aminor proportion of aromatic hydrocarbons (preferably less than 30%,more particularly, less than 15% by weight of the oil). Alternatively,the oil may be totally non-aromatic. Suitable oligomers includepolypropylenes, polybutenes, hydrogenated polyisoprene, hydrogenatedpolybutadiene, or the like having average molecular weights betweenabout 350 and about 10,000. Preferred are LUMINOL T350, a mineral oilavailable from Petrocanada and KAYDOL OIL available from WitcoCorporation.

The hot melt adhesive of the present invention also comprises 0 to 3weight percent, preferably, 0.3 to 3.0 weight percent, of anantioxidant. Among the applicable stabilizers or antioxidants includedherein are the hindered phenols or hindered phenols in combination witha secondary antioxidant such as distearyl thiodipropionate (“DSTDP”) ordilauryl thio-dipropionate (“DLTDP”). Hindered phenols as used hereinare as phenolic compounds containing sterically bulky radicals in closeproximity to the phenolic hydroxyl group thereof. The presence of thesesterically bulky substituted radicals in the vicinity of the hydroxylgroup serves to retard its stretching frequency and, correspondingly,the reactivity; this steric hindrance provides the phenolic compoundwith its stabilizing properties. Representative hindered phenolsinclude: 1,3,5-trimethyl 2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene; pentaerythrityltetrakis-3(3,5-di-tert-butyl-4-hydroxyphenyl)propionate; pentaerythritoltetrakis (3-lauryl thiodipropionate);n-octadecyl-3,5-di-tert-butyl-4-hydroxyphenol)-propionate;4,4′-methylenebis (2,6-tert-butylphenol); 4,4′-thiobis(6-tert-butyl-o-cresol); 2,6-di-tertbutylphenol;6-(4-hydroxyphenoxy)-2,4-bis(n-octyl-thio)-1,3,5-triazine;di-n-octadecyl 3,5-di-tert-butyl-4-hydroxy-benzyl-phosphonate;2-(n-octylthio)ethyl 3,5-di-tert-butyl-4-hydroxy-benzoate; and sorbitolhexa[3-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate]. Preferredantioxidants are SUMILIZER TDP, a secondary antioxidant available fromSumitomo Chemical Company and IRGANOX 1010 a hindered phenol primaryantioxidant available from Ciba-Geigy. The stabilizer is present inamounts of 0.3 to 3% by weight, preferably 0.3 to 1.5%, most preferably0.8%.

Optional additives may be incorporated into the hot melt compositionsdepending on the end use of the composition. Among these additives maybe included colorants such as titanium dioxide; and fillers such as talcand clay, etc., as well as minor amounts (e.g., less than about 5%) of apetroleum derived wax.

One embodiment of the present invention is a multipurpose hot meltadhesive comprising:

(a) 5 to than 35 weight percent of a styrene-butadiene-styrene blockcopolymer with molecular weight such that at 25% in toluene theviscosity is greater than 1000 cPs;

(b) 2 to 30 weight percent endblock resin prepared from anysubstantially aromatic monomers having a polymerizable unsaturatedgroup;

(c) 30 to 70 weight percent of a solid tackifier;

(d) 0 to 25% of a compatible non-functionalized block copolymer;

(e) 0 to 30 weight percent oil; and

(e) 0 to 3 weight percent antioxidant wherein the tackifier is presentin an amount greater than the block copolymer and wherein the adhesivehas a viscosity less than 35,000 cPs at 325° F. and a cohesive strengthmeasured in Hang-Bead time of greater than 45 seconds.

In another embodiment the adhesive comprises:

(a) 5 to than 35, preferably 10 to 35, weight percent of astyrene-butadiene-styrene block copolymer with molecular weight suchthat at 25% in toluene the viscosity is greater than 2000 cPs, and lessthan 32% styrene;

(b) 2 to 30 weight percent of the endblock resin;

(c) 30 to 70 weight percent of a solid tackifier;

(d) 0 to 30 weight percent oil; and

(e) 0 to 3 weight percent antioxidant wherein the tackifier is presentin an amount greater than the block copolymer and wherein the adhesivehas a viscosity less than 35,000 cPs at 325° F. and a cohesive strengthmeasured in Hang-Bead time of greater than 45 seconds.

In a further embodiment the adhesive comprises:

(a) 10 to 25 weight percent of a styrene-butadiene-styrene blockcopolymer with molecular weight such that at 25% in toluene theviscosity is greater than 2000 cPs, and less than 32 weight percentstyrene;

(b) 5 to 20 weight percent endblock resin;

(c) 45 to 65 weight percent of a tackifier;

(d) 0 to 30 weight percent oil; and

(e) 0 to 3 weight percent antioxidant.

In a preferred embodiment, the present invention is directed to amultipurpose hot melt adhesive comprising:

(a) 15 to 20 weight percent of a styrene-butadiene-styrene blockcopolymer with molecular weight such that at 25% in toluene theviscosity is greater than 2000 cPs and 32 weight percent styrene;

(b) 5 to 20 weight percent aromatic resin;

(c) 45 to 65 weight percent of a solid tackifier;

(d) 5 to 20 weight percent oil; and

(e) 0.3 to 1.5 weight percent antioxidant wherein the tackifier ispresent in an amount greater than the block copolymer and wherein theadhesive has a viscosity less than 35,000 cPs at 325° F. and a cohesivestrength measured in Hang-Bead time of greater than 45 seconds.

The resultant adhesives may be used in the assembly or construction ofvarious disposable articles including, but not limited to, disposablediapers, disposable feminine products, adult incontinent products,hospital gowns, bed pads and the like. In particular, adhesives areuseful for the assembly of disposable articles wherein at least onepolyethylene or polypropylene substrate is bonded to at least onetissue, nonwoven, polyethylene or polypropylene substrate. In addition,the adhesives are useful in the bonding of elastic to polyethylene,polypropylene or nonwoven substrate so as, for example, to impartelongation resistant gathers thereto. The adhesive may also be utilizedin less demanding disposable construction applications such as for endor perimeter sealing.

The use of high molecular weight copolymers allows the use of a smalleramount of polymer, resulting in a less stiff adhesive formulationdesirable for a pressure sensitive adhesive. The adhesive of the presentinvention will have a stiffness less than 15×10⁵ dyne/cm² at 40° C.,preferably less than 10×10⁵ dyne/cm² at 40° C.

Low stiffness, combined with high loss modulus, leads to high loop tack.Loop tack is a measure of an adhesive's grab. High loop tack indicatesthat the adhesive will form a good pressure sensitive bond with lowapplied pressure. A pressure sensitive hot melt adhesives, in accordancewith the present invention, will display loop tack values preferablygreater than 50 oz/in², most preferably 90-200 oz/in².

The viscosity of the resulting adhesive formulation will be less than35,000 cPs at 325° F., allowing for use with most conventional adhesiveapplication machinery. In a more preferred embodiment, the viscosity ofthe adhesive formulation of the present invention will be less than15,000 cPs, providing the adhesive with sprayability and processability.

A preferred composition for a hot melt adhesive in accordance with thepresent invention, which is sprayable and suitable for elasticattachment, will have a viscosity less than 35,000 cPs at 325° F. and acohesive strength measured in Hang-Bead time of greater than 45 seconds,and preferably the SBS block copolymer comprises less than 35% styrene.

A preferred composition for a multipurpose adhesive in accordance withthe present invention, will have a viscosity less than 35,000 cPs at325° F., a cohesive strength measured in Hang-Bead time of greater than45 seconds, a loop tack greater than 50 oz/in², a stiffness less than15×10⁵ dyne/cm² at 40° C., and preferably the SBS block copolymercomprises less than 35% styrene.

The adhesive formulations of the present invention, in addition to beingsuitable for elastic attachment, have a long open time and high pressuresensitivity allowing them to be very good as a construction adhesive. Inaddition, the adhesive of the present invention will have good cohesivestrength as measured by Hang-Bead time. Hang-Bead time is a relativemeasure of cohesive strength; the adhesives of the present inventionwill have a Hang-Bead time of at least 45 seconds, preferably greaterthan 150 seconds, or most preferably greater than 500 seconds.

The following examples are merely illustrative and not intended to limitthe scope of the present claims in any manner.

EXAMPLES

The adhesives of the invention were prepared using the followingprocedure:

In a sigma blade mixer, heated to about 163° C., the rubber was chargedalong with the antioxidants, tackifying resin and mineral oil. Therubber to plasticizer ratio (oil+resin) was about 1:1.5. Mixing of thecontents was continued until a homogeneous mixture was obtained. At thispoint, the remainder of the resin was added slowly followed by additionof the remainder of the mineral oil. Mixing was then continued for about10 more minutes. The molten adhesive was then poured into a siliconerelease-coated container and allowed to cool to room temperature.

The following copolymers were used:

Rubber *Solution Viscosity, cPs Styrene Diblock % % Structure Sol T 6414350 40 25 Branched Sol T 6302 4,000 31 10 Linear D1184 20,000 31 16Branched DPX 563 7970 31 0 Linear D1122X 730 38 10 Branched *Neatpolymer concentration, 25% w in Toluene @ 73° F.

Sol T 6414 is available from EniChem Americas (Agip USA Inc.) and D1122Xis available from Shell Chemical Company.

The following tests were performed on the adhesives to determine theviscosity, cohesive strength, peel strength, tack, and stiffness:

Viscosity

Viscosity measurements were made in a Brookefield viscometer at 325° F.and are shown in cPs units.

Cohesive Strength Measurements

It is generally recognized that to hold the elastics in place indisposable articles, an adhesive with a high resistance to creep underthe stress of the elastic, or high “cohesive strength,” is required.Various measures of cohesive strength have been used in the past. Here,cohesive strength was measured using a creep test on a single bead ofadhesive. The bead is applied to a plate at 0.53 grams/meter. The platecontains a slit cut transverse to the direction in which the bead isapplied. Each slit is ⅛″ wide and extends to one side of the plate. Thebead of adhesive is applied to the plate and bridges over the slit, suchthat it does not droop into the slit itself. The plate is then placed inan oven at 105° F. and allowed to equilibrate for 20 min. A 40 gramweight attached to a paper clip is then hung on the bead of adhesive inthe section over the slit using the other end of the paper clip (size #1GEM CLIPS available form BT Office Products International). The timerequired for the adhesive bead to break is termed the “Hang-Bead time”.

Peel Strength Measurements

Peel strength was measured by performing a 180° peel test on highdensity polyethylene, “HDPE”, substrate using an Instron. About 2 to 3mil thick coating of the adhesive is applied on to Mylar film usingheated rollers and bonded to silicone coated release paper. Threespecimens each 3″×1″ in dimensions are cut perpendicular to the machinedirection from the coated Mylar. After conditioning overnight at 72° F.and 50% relative humidity, the release paper is removed and thespecimens are bonded to a HDPE plate. The bonds are then rolled using a4.5 lb roller. After conditioning the bonds for about half-hour, theyare peeled in the Instron at 2″/minute. The HDPE plate is in thestationary jaw, and the Mylar is in the movable jaw. The results arereported as an average load in grams.

Loop Tack Measurements

Loop tack is measured using the TMI Loop Tack tester made by cutting5″×1″ dimensions specimens of the Mylar coated laminates along themachine direction. After conditioning overnight at 72° F. and 50%relative humidity, the laminate is folded into a loop by taping the endstogether. The loop is then mounted on the Loop tack tester and astainless steel plate is clamped to the base of the tester. When thetest is started, the loop is brought in contact with the Stainless Steelplate and then withdrawn. The load it takes to withdraw from the plateis recorded as the loop tack in oz/in².

Rheology Study

A Rheometrics Dynamic Mechanical Analyzer (model RDA 700) was used toobtain the elastic (G′) and loss (G″) moduli versus temperature. Theinstrument was controlled by Rhios software version 4.3.2. Parallelplates 8 mm in diameter and separated by a gap of about 2 mm were used.The sample was loaded and then cooled to about −100° C. and the timeprogram started. The program test increased the temperature at 5° C.intervals followed by a soak time at each temperature of 10 seconds. Theconvection oven containing the sample was flushed continuously withnitrogen. The frequency was maintained at 10 rad/s. The initial strainat the start of the test was 0.05% (at the outer edge of the plates.) Anautostrain option in the software was used to maintain an accuratelymeasurable torque throughout the test. The option was configured suchthat the maximum applied strain allowed by the software was 80%. Theautostrain program adjusted the strain at each temperature increment ifwarranted using the following procedure. If the torque was below 200g-cm, the strain was increased by 25% of the current value. If thetorque was above 1200, it was decreased by 25% of the current value. Attorques between 200 and 1200 g-cm, no change in strain was made at thattemperature increment. The shear storage or elastic modulus (G′) and theshear loss modulus (G″) are calculated by the software from the torqueand strain data. Their ratio, G″/G′, also known as the tan delta, wasalso calculated. The temperature corresponding to the tan δ peakassociated with the midblock is reported as the midblock Tg.

In the following tables, stiffness values, G′, are reported in units of10⁵ dyne/cm² at 40° C.

Open Time Measurements

Open time of the adhesives was measured by bonding polyethylene topolyethylene in a Kanebo bond tester. A known amount of adhesive at 350°F. is applied as a bead on a polyethylene film mounted on a plate. Theopen time is adjusted in the bond tester to any desired value. After theopen time elapses, the tester bonds another sheet of polyethylene,mounted on a plate, on to the adhesive bead under a constant load. Thebead width is measured as a function of open time. The open time of theadhesive is taken as the time when the bead width changes from greaterthan or equal to 2 mm to less than 2 mm.

Example 1

Table I shows the effect of endblock resins on the cohesive strength ofthe adhesives. Samples I-1, I-2, I-3, I-4 were prepared using thegeneral mixing procedure described earlier using 18 weight percent SOLT6302 and 16 weight percent oil in Sample I-1 and 18 weight percent oilin Samples I-2, I-3 and I-4. In each sample, the tackifier used is aC₅/C₉ hydrocarbon resin derived from petroleum with a Ring and Ball of100 to 105° C.

It can be seen that the cohesive strength (Hang-Bead time) of Sample 1,which does not contain endblock, is significantly lower than SamplesI-2, I-3, I-4 due to the absence of endblock resin. The cohesivestrength is also dependent on the amount of endblock resin added. Thisis evident in Samples I-2, I-3, I-4, which show an increase in cohesivestrength with increasing amount of endblock resin. In multipurposeapplications, which includes both elastic attachment and construction,it is desirable to have high peel and cohesive strengths withconsiderable tack or pressure sensitivity. It is well known that it isdifficult to optimize cohesive and peel strengths without altering oneor the other. Sample I-4 in Table I has high cohesive strength but verylow peel strength. In addition, the sacrifice for this high cohesivestrength of Sample I-4 is the high stiffness and very low tack. WhileSample I-4 may be suitable for some elastic applications, it would makea poor construction adhesive due to its lack of pressure sensitivity.

TABLE I Amt. Visc. @ 325° F. Hang-Bead time Peel Tack Stiffness @ 40° C.Resin (wt. %) (cPs) (s.) (g.) (oz/in²) (10⁵ dyne/cm²) I-1 Tackifier 66 8,450  117 2582 191 3.0 Hercolite 240  0 I-2 Tackifier 54 10,050  9101876 101 3.8 Hercolite 240 10 I-3 Tackifier 49 10,700 1074 1129  78 5.5Hercolite 240 15 I-4 Tackifier 44 14,700 2310  625  3 40 Hercolite 24020

Example 2

Table II demonstrates that different high molecular weight rubbers canbe used to achieve a balance between cohesive strength and peelstrength. Each sample was prepared with 18 weight percent rubber; 54weight percent of the tackifier used in Example I; 10 weight percentHercolite 240; and 18 weight percent oil.

TABLE II Visc. @ 325° F. Hang-Bead Peel Tack Stiffness @ 40° C. Polymer(cPs) (s.) (g.) (oz/in²) (10⁵ dyne/cm²) II-1 Sol T 6302 10,050 910 1876101 3.8 II-2 DPX 563 13,920 784 1780 111 3.4 II-3 D1184 26,050 3,050  1476 108 2.9

Example 3

Table III compares samples made from prior art to that of the presentinvention. Sample III-1 is made using the example in Raykovitz, U.S.Pat. No. 4,944,993. Sample III-2 is made using the preferred example ofMalcolm, EP 0 368 141 A2. Samples III-1 and III-2 have very poorcohesive strength compared to Sample III-4 corresponding to the presentinvention. Sample III-3 shows that by adding endblock resin to thepreferred example of Malcolm's patent, it is possible to improve thecohesive strength. Although Raykovitz patent example contains endblockresin, it does not demonstrate the effect of endblock resin on cohesivestrength and peel strength. This invention clearly demonstrates theeffective use of endblock resins in combination with high molecularweight rubbers to optimize peel and cohesive strengths.

The C₅/C₉ resin used in Samples III-1 and III-4 are a C₅/C₉ hydrocarbonresin derived from petroleum with a Ring and Ball softening pointbetween 100-105° C. UnitacR100L is a pentaerythritol ester of rosinavailable from Union Camp. Zonatac 105 is a styrenated terpene availablefrom Arizona Chemical Company.

TABLE III Amt. Amt Oil Visc. @ 325° F. Hang-bead Peel Tack Stiffness @40° C. Polymer (wt. %) Resin (wt. %) (wt. %) (cPs) time (s.) (g.)(oz/in²) (10⁵dyne/cm²) III-1 D1122 15 C₅/C₉ resin 52 25   835 11 868 962.7 Kristalex 5140 8 III-2 D1184 10.64 Unitac R100L 53.3 19.6* 1400 02026 49 0.9 Zonatac 105 15.96 III-3 D1184 10.64 Unitac R100L 43.3 19.6*1,950 27 2260 8 1.7 Zonatac 105 15.96 Hercolite 240 10.0 III-4 Sol T6302 18 C₅/C₉ resin 54 18   10,050 910 1876 101 3.8 Hercolite 240 10

Example 4

The glass transition temperature, Tg, of the midblock (butadiene in thisapplication) is an important parameter, which determines the stiffnessand the peel strength of the adhesive at room temperature. In addition,it can be used to study the effect of endblock resins in hot meltadhesives. When the midblock resin is replaced in part by the endblockresin, the Tg of the midblock is expected to decrease, provided theendblock does not have any association with the midblock. Also, thebenefit of adding an endblock is achieved only when it associates withthe endblock of the block copolymer rubber. In SBS rubbers, due to thecloseness in solubility parameters of styrene and butadiene, which areabout 8.9 and 8.4, respectively, the endblock resins tend to associatewith the midblock too. This association is readily seen as an increasein midblock Tg on adding the endblock resin. The extent of thisassociation is dependent on the molecular weight of the rubber and thesoftening point of the endblock resin.

Table IV compares the midblock Tg in low molecular weight and highmolecular weight SBS rubber formulations. Samples IV-1 and IV-2 wereprepared according to Raykovitz, U.S. Pat. No. 4,944,993. In each of thesamples in Table IV, the tackifier is a C₅/C₉ hydrocarbon resin derivedfrom petroleum with a Ring and Ball softening point between 100 to 105°C.

Sample IV-4 is an example of the present invention. It can be seen thatthe increase in midblock Tg on adding endblock resin is much higher in alow molecular weight SBS, Sol T 6414 (Sample IV-2), than in a highmolecular weight SBS, Sol T 6302 (Sample IV-4). The consequence of thisgreater association with the midblock in Sol T 6414 can be readily seenin the cohesive strength. The cohesive strength increase on addingendblock resin, is much lower in Sol T 6414 (Sample IV-2) than in Sol T6302 (Sample IV-4). Hence, by using a high molecular weight rubber incombination with endblock resin, it is possible to achieve high cohesivestrength at low levels of rubber.

TABLE IV Amt. Amt. Oil Midblock Tg ΔTg Hang-Bead time, Sample Polymer(wt. %) Resin (wt. %) (wt. %) (° C.) (° C.) (s.) IV-1 Sol T 6414 30tackifier 59 11 20 — 310 Hercolite 240  0 IV-2 Sol T 6414 30 tackifier49 11 27 7   537 Hercolite 240 10 IV-3 Sol T 6302 18 tackifier 66 16 19— 117 Hercolite 240  0 IV-4 Sol T 6302 18 tackifier 56 16 21 2.0 943Hercolite 240 10

Example 5

In hot melt adhesives applications, it is desirable to have long opentime. Open time of the adhesive can be broadly defined as the longesttime one can wait after applying the adhesive before bonding and stillobtain an acceptable bond. Table V compares the adhesive of presentinvention (Sample V-2) to an adhesive prepared according to Raykovitz,U.S. Pat. No. 4,944,993, with higher amount of rubber (Sample V-1). Thehigher amount of rubber in Sample V-1 is to achieve sufficient cohesivestrength. It can be seen that the open time in Sample V-1 is much lowerthan that of Sample V-2. The longer open time is another benefit ofadding lower amounts of high molecular weight rubber while maintaininggood cohesive strength.

In each of the samples in Table V, the tackifier is a C₅/C₉ hydrocarbonresin derived from petroleum with a Ring and Ball softening pointbetween 100 to 105° C.

TABLE V Amt. Amount Oil Visc. @ 325° C. Open time, Hang-bead time,Sample Polymer (wt. %) Resin (wt. %) (wt. %) (cPs) (s.) (s.) V-1 D1122X30 tackifier 59 11 10,650 1.0 517 V-2 Sol T 6302 18 tackifier 54 1810,050 2.0 917 Hercolite 240 10

What is claimed is:
 1. A hot melt adhesive comprising: (a) 5 to 35weight percent of a styrene-butadiene-styrene block copolymer withmolecular weight such that at 25% in toluene the viscosity is greaterthan 1000 cPs wherein the styrene-butadiene-styrene block copolymercomprises less than 35% styrene; (b) 2 to 30 weight percent endblockresin prepared from any substantially aromatic monomers having apolymerizable unsaturated group; (c) 30 to 70 weight percent of a solidtackifier; (d) 0 to 25% of a compatible non-functionalized blockcopolymer; (e) 0 to 30 weight percent oil; and (f) 0 to 3 weight percentantioxidant wherein the tackifier is present in an amount greater thanthe block copolymer and wherein the adhesive has a viscosity less than35,000 cPs at 325° F. and a cohesive strength measured in Hang-Bead timeof greater than 45 seconds.
 2. A hot melt adhesive consistingessentially of: (a) 5 to 35 weight percent of astyrene-butadiene-styrene block copolymer with molecular weight suchthat at 25% in toluene the viscosity is greater than 1000 cPs whereinthe styrene-butadiene-styrene block copolymer comprises less than 35%styrene; (b) 2 to 30 weight percent endblock resin prepared from anysubstantially aromatic monomers having a polymerizable unsaturatedgroup; (c) 30 to 70 weight percent of a solid tackifier; (d) 0 to 25% ofa compatible non-functionalized block copolymer; (e) 0 to 30 weightpercent oil; and (f) 0 to 3 weight percent antioxidant wherein thetackifier is present in an amount greater than the block copolymer andwherein the adhesive has a viscosity less than 35,000 cPs at 325° F. anda cohesive strength measured in Hang-Bead time of greater than 45seconds.
 3. The hot melt adhesive according to claim 1 wherein theadhesive has a loop tack greater than 50 oz/in² and a stiffness lessthan 15×10⁵ dyne/cm² at 40° C.
 4. The hot melt adhesive of claim 1wherein the endblock resin is selected from the group consisting ofstyrene, alphamethyl styrene, vinyl toluene, methoxy styrene, tertiarybutyl styrene, chlorostyrene, coumarone, indene, and methyl indene.
 5. Ahot melt adhesive consisting essentially of: (a) 5 to 35 weight percentof a styrene-butadiene-styrene block copolymer with molecular weightsuch that at 25% in toluene the viscosity is greater than 2000 cPs, andwherein the styrene-butadiene-styrene block copolymer comprises lessthan 32% styrene; (b) 2 to 30 weight percent endblock resin preparedfrom any substantially aromatic monomers having a polymerizableunsaturated group; (c) 30 to 70 weight percent of a solid tackifier; (d)0 to 25% of a compatible non-functionalized block copolymer; (e) 0 to 30weight percent oil; and (f) 0 to 3 weight percent antioxidant; whereinthe tackifier is present in an amount greater than the copolymer andwherein the adhesive has a viscosity less than 35,000 cPs at 325° F. anda cohesive strength measured in Hang-Bead time of greater than 45seconds.
 6. A hot melt adhesive according to claim 3 wherein theadhesive has a loop tack greater than 50 oz/in² and a stiffness lessthan 15×10⁵ dyne/cm² at 40° C.
 7. A hot melt adhesive consistingessentially of: (a) 10 to 25 weight percent of astyrene-butadiene-styrene block copolymer with molecular weight suchthat at 25% in toluene the viscosity is greater than 2000 cPs, andcomprises less than 32 weight % styrene; (b) 5 to 20 weight percentendblock resin prepared from any substantially aromatic monomers havinga polymerizable unsaturated group; (c) 45 to 65 weight percent of asolid tackifier; (d) 0 to 25% of a compatible non-functionalized blockcopolymer; (e) 0 to 30 weight percent oil; and (f) 0 to 3 weight percentantioxidant wherein the adhesive has a viscosity less than 35,000 cPs at325° F. and a cohesive strength measured in Hang-Bead time of greaterthan 45 seconds.
 8. A hot melt adhesive according to claim 5 wherein theadhesive has a loop tack greater than 50 oz/in² and a stiffness lessthan 15×10⁵ dyne/cm² at 40° C.
 9. A hot melt adhesive consistingessentially of: (a) 15 to 20 weight percent of astyrene-butadiene-styrene block copolymer with molecular weight suchthat at 25% in toluene the viscosity is greater than 2000 cPs, andcomprises less than 32% styrene; (b) 5 to 20 weight percent endblockresin prepared from any substantially aromatic monomers having apolymerizable unsaturated group; (c) 45 to 65 weight percent of a solidtackifier; (d) 0 to 25% of a compatible non-functionalized blockcopolymer; (e) 5 to 20 weight percent oil; and (f) 0.3 to 1.5 weightpercent antioxidant wherein the adhesive has a viscosity less than35,000 cPs at 325° F. and a cohesive strength measured in Hang-Bead timeof greater than 45 seconds.
 10. A hot melt adhesive according to claim 7wherein the adhesive has a loop tack greater than 50 oz/in² and astiffness less than 15×10⁵ dyne/cm² at 40° C.
 11. A disposable productbonded utilizing the hot melt adhesive composition of claim
 1. 12. Adisposable product comprising a hot melt adhesive bonded utilizing thehot melt adhesive composition of claim 2.